The Long-Term Soil Productivity Experiment

The Long-Term Soil Productivity Network

The Long-Term Soil Productivity network of experiments began in 1989 as a "grass roots" proposal that grew to a national program of the USDA Forest Service. LTSP was founded to examine the long-term consequences of soil disturbance on fundamental forest productivity. The concept caught the imagination of others. Soon, partnerships and affiliations were forged among public and private sectors in the United States and Canada. Today, more than 100 LTSP and affiliated sites comprise the world's largest coordinated research network addressing basic and applied science issues of forest management and sustained productivity. Studies range from elucidating mechanisms controlling carbon capture above and below ground, to developing indices of soil quality practicable in monitoring.

The LTSP Principle

The capacity of a forest site to capture carbon and convert it into biomass defines fundamental site productivity. The National Forest Management Act of 1976 (NFMA) mandates that this capacity must be protected on federally managed lands. Responding to NFMA, the USDA Forest Service began a soil-based monitoring program for its managed forests. Lacking an extensive research base, soil-based standards were predicated largely on professional judgment. To provide a stronger foundation based on fundamental science, a national program of Long-Term Soil Productivity research (LTSP) was established. LTSP is predicated on the principle that within the constraints of climate, a site's potential net primary productivity is strongly controlled by physical, chemical, and biotic soil processes affected readily by management. The key properties affected directly by management are soil porosity and site organic matter. Together, these properties regulate critical site processes through their roles in microbial activity, soil aggregate stability, water and gas exchange, physical restrictions on rooting, and resource availability.

The Experiment

Soil porosity and site organic matter are affected by any forest management activity disturbing the soil. The question is "how much soil disturbance is too much?" The following four hypotheses are central to the LTSP experiment.

Null hypothesis

Alternative hypothesis

1. Pulse changes in site organic matter and/or soil porosity do not affect the sustained productive potential of a site (sustained capacity to capture carbon and produce phytomass).

2. If impacts on productivity occur from changes in organic matter and porosity, they are universal.

The biological significance of a change in organic matter or porosity varies by climate and soil type.

3. If impacts do occur, they are irreversible.

Negative impacts dissipate with time, or can be mitigated by management practices.

4. Plant diversity has no impact on the productive potential of a site.

Diverse communities affect site potential by using resources more fully or through nutrient cycling changes that affect the soil.

The LTSP program focuses on disturbances associated with timber harvest, but findings apply to any activities altering vegetation or soil. LTSP centers on core experiments that manipulate site organic matter, soil porosity, and the complexity of the plant community. The study was targeted at forest types, age classes, and soil conditions likely to come under active forest management involving harvesting, thinning, or fuel modification. These were fully stocked, young-growth, evenaged forests i.e. not "ancient forests" or nonforested openings. Preliminary plots of 0.2 or 0.4 ha were identified and surveyed for variability in soil and stand conditions. Those with comparable variability were chosen for the experiment. Pretreatment samples then were taken to quantify standing biomass and nutrient capital in the overstory, understory, and forest floor. Stands were then harvested and treatments imposed randomly. Experimental treatments were selected not so much to mimic operational practices of the moment, but rather to bracket the extremes in disturbance likely to occur under present or future management. The main effect treatments are as follows:

We had two reasons for choosing these levels of organic matter manipulation. First, they encompass the extremes in organic matter removal likely under any silvicultural system short of removing surface soil or extracting roots. Second, they produce a step series of nutrient removal that is disproportionate to biomass loss. Experimental treatments were not meant to mimic operational practices, but rather to bracket the extremes in disturbance likely to occur under present or future management. Generally, all factorial combinations of main effect treatments were applied, producing nine core combinations of organic matter removal and soil compaction. Treatment plots (0.4 ha) were separated from residual stands by a distance at least equivalent to the height of bordering trees. This plot size and separation avoided competitive edge effects that could mask the true impact of the treatments, a confounding factor that affects small plot studies and many historical investigations. Following treatment, each plot is regenerated with the same tree species found in the original forest. Each treatment plot is divided in half. On one half, understory vegetation is allowed to develop, producing a diverse plant community. On the other half, understory vegetation is eliminated in order to produce a very simple plant community comprised simply of planted trees. As vegetation develops over time, dry matter production gives a fundamental measure of productivity as affected by soil disturbance and plant community diversity.

Status

The LTSP experiment has been established at 12 locations in California's Sierra Nevada and southern Cascade Range. They represent some of the least to some of the most productive sites in the mixed-conifer forest. Findings from the oldest installations are now available in the scientific literature cited at this website (most notably Powers et al. (in press), Shestak and Busse (in review), Gomez et al. (2000) and Powers (2002). Scientists from several disciplines work on these sites and the study has been the basis for graduate work at four major universities. The installations continue to have educational value for visitors from throughout the world.